The present invention relates to autoregulating electric heaters and more particularly, to an electromagnetic autoregulating electric soldering iron having substantially constant temperature regulation and high efficiency at quite reasonable prices.
In U.S. Pat. No. 4,256,945 of Carter and Krumme, there is described an autoregulating electric heater having a laminated structure; one lamina of which has high magnetic permeability and high resistance and has a low resistance (such as copper) in electrical contact, and therefore, thermal contact with the first lamina. This structure is adapted to be connected across a constant current, a.c., source such that the layers are in a sense in parallel across the source.
Due to skin effect, the current is initially confined to the high magnetic permeability, high resistance layer so that P=KR.sub.1 where P is Power, K is I.sup.2 which is a constant, and R is the effective resistance of the permeable material at high current concentrations. The dissipation of power heats the layer until it approaches its Curie temperature. The permeability of the lamina decreases towards the level of the second layer, copper for instance, at about its Curie temperature. The current is no longer confined to the high resistivity first lamina by the magnetic properties of the first lamina, and spreads into the copper layer; the resistance to the current drops materially, the power consumed, P=KR.sub.2 where R.sub.2 &lt;&lt;R.sub.1, is greatly reduced and the heating effect is reduced to a level that maintains the device at or near the Curie temperature. The device thus thermally autoregulates over a narrow temperature range about the Curie temperature.
The current source employed in the aforesaid patent is typically a high frequency source, to insure that the current is confined to the thin, high resistivity, magnetic layer until the Curie temperature of the magnetic material is attained. Specifically, the maximum regulation is achieved when the thickness of the magnetic layer is of the order of one skin depth at the frequency of operation. Under these circumstances, the maximum change in effective resistance of the structure is achieved at or about the Curie temperature. This fact can be demonstrated by reference to the equation for skin depth in a monolithic, i.e., non-laminar magnetic structure: ##EQU1## where is the resistivity of the material in ohm-cms, is magnetic permeability mu and is frequency of the current. The field falls off in accordance with e.sup.-x where x is thickness/skin depth. Accordingly, in a monolithic structure, by calculation, 63.2% of the current is confined to one skin depth in the high mu material. In the region of the Curie temperature, where .mu.=1, the current spreads into a region ##EQU2## If mu was originally equal to 200 (200-5000 being possible), the skin depth in the region at the Curie temperature increases by the square root of 200; i.e., the skin depth in the monolithic structure is now 14.14 times greater than with .mu.=200.
The same type of reasoning concerning the skin effect may be applied to the two layer laminar structure in the aforesaid patent. Below the Curie temperature, the majority of the current flows in the magnetic layer when the thickness of this layer is nominally one skin depth of the material below the Curie temperature. In the region of the Curie temperature, the majority of the current now flows in the copper and the resistance drops dramatically. If the thickness of this high mu material were greater that two skin depths, the percentage change of current flowing in the high conductivity copper would be less and the resistivity change would not be as dramatic. Similarly, if the thickness of the high mu material were materially less than one skin depth, the percentage of current flowing in the high resistivity material at a temperature less than the Curie temperature would be less so that the change of resistance at the Curie temperature would again not be as dramatic. The region of 0.5 to 1.8 skin depth is preferred.
An exact relationship for the two layer case is quite complex. The basic mathematical formulas for surface impedance from which expressions can be obtained for the ratio of the maximum resistance, R.sub.max, below the Curie temperature, to the minimum resistance, R.sub.min, above the Curie temperature, are given in Section 5.19, pp. 298-303 of the standard reference, "Fields and Waves in Communications Electronics," 3rd Edition, by S. Ramo, J. R. Winnery, and T. VanDuzer, published by John Wiley and Sons, New York, 1965. Although the theory described in the above reference is precise only for the case of flat layers, it is still accurate enough for all practical applications in which the skin depth is substantially less than the radius of curvature.
Difficulty may arise in such devices when the Curie temperature is achieved due to spread of the current and/or magnetic flux into adjacent regions outside of the device, particularly if the device is located close to sensitive electrical components.
In copending patent application of Carter and Krumme, Ser. No. 243,777, filed Mar. 16, 1981, now U.S. Pat. No. 4,701,587 issued Oct. 20, 1987, a continuation-in-part application of the application from which the aforesaid patent matured, there is described a mechanism for preventing the high frequency field generated in the heated device from radiating into the regions adjacent the device. This effect is accomplished by insuring that the copper or other material of high conductivity is sufficiently thick, several skin depths at the frequency of the source, to prevent such radiation and electrical field activity. This feature is important in many applications of the device such as a soldering iron where electromagnetic fields may induce relatively large currents in sensitive circuit components which may destroy such components.
In accordance with the invention of co-pending application of John F. Krumme, Ser. No. 430,317, entitled "Autoregulating Electrically Shielded Heater," filed on Sept. 30, 1982, now abandoned, a continuation-in-part application having been filed and now issued as U.S. Pat. No. 4,695,713, issued Sept. 22, 1987, a relatively low frequency constant current source may be employed as a result of fabricating the normally non-magnetic, low resistivity layer from a high permeability, high Curie temperature material. Thus, the device comprises a high permeability, high resistivity first layer adjacent the current return path and a high permeability, preferably low resistivity second layer remote from the return path; the second layer having a higher Curie temperature than the first-mentioned layer.
The theory of operation underlying the invention of the aforesaid application filed on Sept. 30, 1982, now U.S. Pat. No. 4,695,713, is that by using a high permeability, high Curie temperature material as the low resistivity layer, the skin depth of the current in this second layer is such as to confine the current to a quite thin layer even at low frequencies thereby essentially insulating the outer surfaces electrically and magnetically but not thermally with a low resistivity layer of manageable thickness. The second layer is preferably formed of a low resistivity material, but this is not essential.
The power regulation ratios (.DELTA.R) in such a device; 2:1 to 4:1, are not as high as with the device of the patent with a resistivity difference of about 10:1, but the .DELTA.R difference may be reduced by using materials of higher and lower resistivities for the lower Curie temperature and high Curie temperature materials, respectively. Also, a high mu, relatively low resistivity material such as iron or low carbon steel may be employed to further increase the power regulation ratio.
In accordance with the invention of co-pending patent application Ser. No. 445,862 of John F. Krumme filed on Dec. 1, 1982, autoregulating power ratios of 6:1 to 7:1 are attained while retaining the ability to utilize low frequency supplies without producing unacceptable levels of field radiation.
The objects of the invention are achieved by providing a region of high conductivity at the interface of the two members having high permeability as set forth in the Krumme application, Ser. No. 430,317, filed on Sept. 30, 1982 now U.S. Pat. No. 4,695,713.
The material in the interface region may be copper, for instance, or other highly conductive material. The material may appear as a separate layer, a sandwich of magnetic, non-magnetic and magnetic material or may be bonded to the high and/or low Curie temperature, ferromagnetic layers at the interface to provide a low resistivity, interface region.
With autoregulating ratios of 6:1 and 7:1, the heating variations below and above Curie temperature are quite large so that the apparatus may respond rapidly to thermal load variations and thus maintain accurate temperature regulation in a small device operating at low frequency.
The disclosure of the above cited applications are incorporated herein by reference, all being assigned to the same assignee as the present invention.
A difficulty is encountered when any of the above heaters are employed in soldering irons such as illustrated in FIG. 4 of the aforesaid patent. The impedance of the soldering iron, due to its relatively small size, is quite low (of the order of 0.1 to 0.25 ohm) and in consequence, presents a poor impedance match to the source. This problem is mitigated to some extent by including impedance matching circuits in the handle of the soldering iron. In such a case; however, a greater resistance appears in the handle than at the tip of the iron, making the handle quite hot and the overall soldering iron performance quite inefficient; i.e., more energy is dissipated in the handle than in the iron.